Technical Field
The invention relates to a test piece for determining a specific material property of a fiber-reinforced plastic composite under applied mechanical loading. The invention also relates to a test method and to a use of the test piece for testing a fiber-reinforced plastic composite of a component of a wind turbine generator system, in particular a rotor blade.
Description of the Related Art
Various fiber-reinforced plastic composites are used in technical applications. The fiber-reinforced plastic composites concerned consist of fibers and a corresponding matrix, which provides the necessary bonding of the composite. The profile of properties of these fiber-reinforced plastic composites is determined not only by the selection of fibers and matrix material but also by the orientation of the fibers in the textile fabric. The materials occurring in the composite usually have functional properties that are for a specific purpose within their area of use. Substance-related properties, under some circumstances also geometrical properties, of the individual components are of importance for the properties of the material or composite component obtained. Composite components usually have properties that represent an optimized behavior of the molded part under the effect of a load. The properties may be assigned to a certain strength or a certain stiffness or a certain extensibility with regard to applied mechanical loads.
A pre-requisite for a fiber-reinforced plastic composite component is that, under the effect of a load, the behavior of the composite represents an optimization with respect to the individual components. The development is heading toward optimizing the required properties in combination with the service life in order to withstand cyclical loading over many years.
An example of this is a corresponding fiber-reinforced component of a rotor blade of a wind turbine generator system. Rotor blades are generally made up of appropriate fibers, i.e., primarily glass and/or carbon fibers, in a resin-like LamiMatrix material. These fibers are generally oriented in or along the longitudinal axis of the rotor blade, the exact alignment of the fibers usually being very difficult to control, in dependence on the production process. In the case of rotor blades of wind turbine generator systems, primarily great load effects are exerted. Under these loads, the rotor blades should withstand both the static loading and the very frequent dynamic loading.
Materials testing generally replicates individual stress scenarios on standardized test pieces. On account of the great directional dependence of the properties, the various types of stressing are carried out with different specimens or test pieces longitudinally and transversely in relation to the main direction of the fibers. Apart from the international standardization, tests are described in various national or regional standards, as well as in company-own codes of practice. This produces a broad purview, describing around 20 generic test methods.
The testing on components, parts of structures and complete structures is generally based to a great extent on the stresses or loads that occur in later operation. Strengths, energy absorption, material fatigue and lifetime expectancy are at the forefront. On account of the directional sensitivity and shear sensitivity of the fiber composites, testing forces should as far as possible be applied in the direction intended. The axial error is referred to as misalignment and is subject to narrow limits. Special measuring devices are based on the form and size of the test piece and offer additional supporting and holding devices to avoid classic failure due to misalignment or Euler's buckling. The alignment is performed here by means of mechanical adjusting devices. The force introduction into the test piece should take place over a large surface area, which is achieved by means of force introduction elements. Specimen holders, which operate on the wedge or wedge-screw principle, are usual for this. The exact alignment can be controlled in a simple case with the aid of a strain gage. This is correspondingly applied to the test piece and verified by the measured strain of the specimen.
The measurement or determination of specific material properties with regard to the mechanics for rotor blades or rotor blades in wind turbine generator systems presupposes a high load effect. For the determination of the necessary material properties, the corresponding forces should act on the test piece. The devices for the determination are usually provided with additional buckling support, in order to prevent device- and test-method-specific failure of the component. The often occurring buckling means that there is an abrupt or violent failure of the test pieces and, in principle, does not provide the structure-specific result for the respective fiber-reinforced plastic composite. The attached buckling supports or holding devices have the effect that the axial alignment is ensured and buckling prevented, since corresponding compressive forces act against the bending moment that leads to buckling.
The German Patent and Trademark Office has searched the following prior art in the priority application: DE 10 2006 035 274 A1, DE 10 2010 002 131 A1 and DE 20 2012 008 324 U1.